1. Field of the Invention
The invention relates to a device for detecting flow rates and particularly to a flow sensor in an internal combustion engine or a flow sensor used in fuel cell systems.
2. Background Art
Air flow sensors are disposed in the intake air passage of internal combustion engines, such as those in automobiles, for measuring the intake air amount. Thermal types are becoming more and more mainstream because of their ability to directly detect mass air amounts. In recent years, particularly, those airflow sensors manufactured with the semiconductor micromachining technology have been gaining attention because of their fast response and their ability to detect backflow by utilizing such fast response.
A conventional example of technology related to thermal airflow sensors utilizing semiconductor substrates is disclosed in Patent Document 1. In this example, a heating resistor disposed between an upstream temperature-measuring resistor and a downstream temperature-measuring resistor is heated by a current that is caused to flow therethrough, whereby a flow rate signal based on the difference between the outputs of the upstream and downstream temperature-measuring resistors is obtained.
In the technique disclosed in Patent Document 1, in order to correct the temperature dependency of the characteristic curve of the thermal airflow sensor in a more preferable manner, the excess temperature of the heat-generating resistor—namely, its temperature difference with respect to the temperature of the measured medium—is adjusted such that it becomes higher as the temperature of the measured medium increases, in consideration of the influence of the temperature dependencies of the thermal conductivity, thermal capacity, and viscosity of the measured medium.
However, in the above conventional example, sufficient consideration is not given to the characteristics (particularly the temperature dependency of resistance values) of the materials of which the upstream and the downstream temperature-measuring resistors are made. Thus, the temperature dependency of the characteristic curve of the thermal airflow sensor is insufficient.
Patent Document 1: JP Patent No. 3342926